1. Field of the Invention
The present invention generally relates to an image signal processing device, and more particularly, to an image contour enhancing device which is capable of improving the resolution at the image contour by processing the color difference signals of signals such as that of an NTSC system and input image signals of video printers.
2. Description of the Prior Art
Recently, accompanying the development of a hard copy technique, particularly a full color hard copy technique, it has become possible to reproduce a high fidelity image by using a copy-printing technique such as a sublime type thermal copying technique. In respect of the color reproduction, a reproducing ability equivalent to the silver print has been reached by dint of a recording material and the image processing. In respect of the resolution, the level of the silver print is being approached by using a highly minute image signal such as a high-vision signal.
However, in a video printer for recording TV signals of the current system, even if the resolution of the printer is high, the resolution thereof is restricted by the band restriction of the image signal in an NTSC system, etc., and therefore in the present circumstance, a resolution sufficient for the recording image can not be obtained. Particularly, in the color difference signal of the image signal, since the bandwidth thereof is considerably limited as compared with the brightness signal, the horizontal resolution is extremely low such as several tens of lines, there has been such a problem that color blurring is caused in the record image.
Conventionally, the apparent resolution of the image has been improved by way of, for example, (1) a method of raising the gain of the high band frequency, or (2) a method of emphasizing the contour by taking a Laplacian of the second order differentiation.
Method (1) of raising the gain of the high band frequency is literally a method of raising the gain of the high band part as compared with the low band part so as to emphasize the image of the high band part.
Method (2) of emphasizing the contour will be described with reference to FIG. 8.
In FIG. 8, curve (a) shows a waveform of an input image signal, curve (b) shows a differential waveform representing the contour of the image by differentiating the curve (a), and the curve (c) is a second order differential waveform obtained by further differentiating the curve (b). The second order differential waveform formed in this manner corresponds to higher harmonic components extracted from the input signal (a). The curve (d) was obtained by reversing the second order differential waveform (c) after a suitable amplification and adding the result to the input image signal (a). The obtained contour-enhanced image signal (d) accompanies preshoots and overshoots at the rising and falling portions which eventually result in enhancing the contour of the image signal, thus improving the apparent resolution.
In the picture image devices such as a television and VCR, the above-mentioned contour enhancing method is arranged to make a second order differential waveform by a delay line or an LC circuit thereby enhancing the contour by an analog circuit. Meanwhile in the digital image processing devices, the method is arranged to obtain the second order differential waveform by the use of a Laplacian operator, etc., as a spacial filter so as to effect the contour enhancing processing. The adding of preshoots and overshoots to the contour of the image signal results in increasing of the amplification of the high frequency component.
Such a contour enhancing method is similar to the unsharp masking conventionally used for a long time in the image reading devices.
However, method (1) has a problem that the S/N ratio is deteriorated because the high band noise component is amplified.
Also, method (2) also results in a problem of deterioration of S/N ratio because the noise signal in the high frequency region is amplified as in method (1).
In addition, although the preshoot and overshoot of the contour part added in the conventional contour enhancing method has an effect of providing accents to the contour so as to improve the resolution using the luminance signal, the preshoots and overshoots adversely affects when the conventional contour enhancing method (2) is used with color data. This is because the added preshoots and overshoots change the hue and tint, resulting in unnatural changes in the color at the contour. For example, by the preshoots and overshoots, a contour changing from red to yellow would results in an unnatural contour changing from red, purple, yellowish green, and yellow, resulting in such a problem that the contour enhancing does not improve the resolution.
Furthermore, in the case of applying the conventional contour enhancing to the contour enhancing of the input signal of an image printer, since the highlight and shade width of the printer is restricted by the color of the recording paper and the maximum recording tint and shade of the printer, the dynamic range thereof is narrower as compared with that of CRT and the like, the preshoot and overshoot are consequently cut off, resulting in a problem that the contour enhancing does not nearly become effective.